Infrared rays are used in most sensors for security or authentication systems, and the optical elements of these sensors are composed of infrared transmitting materials. In recent years, as social awareness grows about security and safety, high performance, small size and versatility are becoming key factors in the design of security and authentication systems. Therefore, increasing attention is being paid to high-performance, small-sized optical elements for compact sensors of the systems and their highly productive manufacturing processes.
Well-known examples of infrared transmitting materials are germanium and zinc selenide. However, these materials, being crystals, can be processed only by grinding forming. Therefore, it is difficult to use these materials to mass-produce complicatedly shaped optical elements such as lens arrays. Particularly, germanium is too expensive to be used for the production of versatile sensors.
One example of a non-crystal infrared transmitting material is chalcogenide glass composed mainly of S, Se and Te. For instance, Patent Documents 1-5 below relate to chalcogenide glass.
Patent Document 1 discloses a method for forming plastics using chalcogenide glass and describes the ways in which to prepare optical elements and the composition of chalcogenide glass suitable for the preparation.
Patent Documents 2 and 3 disclose the composition of chalcogenide glass, but neither of the documents shows the glass composition suitable for mold forming.
Patent Documents 4 and 5 describe methods for mold-forming chalcogenide glass, but neither of them indicates the detailed glass composition. These methods are using kinds of glasses that are difficult to mold-form, and it is necessary to modify the inventions to improve formability.    Patent Document 1: Patent Application Laid-Open No. 2006-290738    Patent Document 2: Patent Application Laid-Open No. 2006-76845    Patent Document 3: Patent Application Laid-Open No. Hei 5-85769    Patent Document 4: Patent Application Laid-Open No. Hei 6-92654    Patent Document 5: Patent Application Laid-Open No. Hei 5-4824